Multi carrier transmission schemes are schemes where a frequency band is segmented into multiple smaller bands (subcarriers) and signals are independently transmitted in the subcarriers. Particularly, an OFDMA (Orthogonal Frequency Division Multiple Access) scheme arranges the subcarriers in such a manner that the individual subcarriers are mutually orthogonal, resulting in higher frequency utilization efficiency for faster and higher capacity transmissions. In the OFDMA scheme, inter-subcarrier interference can be effectively suppressed, and accordingly the individual subcarriers can be used to transmit signals in parallel. As a result, the length of one symbol can be enlarged. Also, it is possible to suppress multipath interference effectively by setting a reasonably long guard interval.
In the MC-CDMA scheme, a transmission signal is code-spread over all the subcarriers. As a result, frequency diversity effect can be improved compared to a simple OFDM scheme, and thus the quality of signal transmissions can be further enhanced.
Meanwhile, in the case where the MC-CDMA scheme and the MIMO scheme are utilized together, signals are code-spread over the overall frequency band, and accordingly a reception signal received at one receive antenna corresponds to a sum signal resulting from multiplexing the spread signals over the overall frequency band corresponding to the number of transmit antennas. In the case where the simple OFDM scheme and the MIMO scheme are utilized together, a certain subcarrier signal is extracted from the sum signal, and signal detection can be performed on the extracted signal component to restore transmission signals transmitted from the respective transmit antennas. However, in the case where the MC-CDMA scheme and the MIMO scheme are utilized together, it is not significant to extract the certain subcarrier signal from the sum signal. This is because the signals are spread over the overall frequency band. If the signals can be despread, a certain signal component may be extracted from the sum signal. However, that is hard because reception signals are generally subject to frequency selective fading and accordingly the orthogonality of the spread codes may be impaired. Thus, it is necessary to simultaneously detect the signals which are spread over the overall frequency band and to which various signals are multiplexed.
As one example, it is assumed that the number of transmit antennas is N, the data modulation level is B (for example, B=4 in 16 QAM), the number of conceived multiplexed codes is P and the signal detection is conducted at the receiver side in accordance with MLD (Maximum Likelihood Detection). (See non-patent document 1 for a conventional QRM-MLD method, for example.) As stated above, the OFDM scheme can suppress the inter-subcarrier interference effectively and restrain the multipath interference within the guard interval sufficiently. In this case, the number of symbol candidates that the receiver side must consider is Equal to 2N×B. On the other hand, in the MC-CDMA scheme, the number of symbol candidates that may have to be considered for the number of multiplexed codes P is equal to 2N×B×P. Since the number of symbol candidates increases exponentially depending on the number of multiplexed codes, the calculation amount of the signal detection significantly increases. In MIMO transmission in accordance with the MC-CDMA scheme, the signal detection can be conducted with a high accuracy, but it is hard to apply the MLD method due to its large calculation amount. On the other hand, in some signal detection methods requiring a less calculation amount such as a ZF (Zero Forcing) method and a MMSE (Minimum Mean Square Error) method, there is a risk of degrading the accuracy of the signal detection. The poor signal detection accuracy at the receiver side means that signals must be transmitted at larger power to maintain required signal quality (required SINR).    Non-patent document 1: K. J. Kim, et al., “Joint channel estimation and data detection algorithm for MIMO-OFDM systems”, Proc. 36th Asilomar Conference on Signals, Systems and Computers, November 2002